Pressure swing adsorption is a useful method of separating the components of gas mixtures when one or more of the components of the mixture are more strongly adsorbed by an adsorbent than are one or more other components of the gas mixture. PSA processes are intermittent in the sense that nonadsorbed gas is produced by passing the feed gas through a bed of the chosen adsorbent until the adsorbent becomes saturated, and then the adsorption bed is taken out of service so that the bed of adsorbent can be regenerated. This is accomplished by depressurizing the bed of adsorbent and, optionally, purging the adsorbent with nonadsorbed gas, to remove the sorbed component from the adsorbent. The regenerated bed of adsorbent is then put back into adsorption service.
In some adsorptive gas separation applications, it is desirable that one or more of the separated components be provided on a substantially continuous basis. This can be conveniently accomplished by using an adsorption system comprising a battery of two or more adsorption vessels operated in parallel and out of phase. This is generally the preferred procedure when the product gas is required on a large volume basis, for example, when more than about 20,000 normal cubic feet per hour (NCFH) of product gas is required. Multiple bed plants are not very attractive for applications in which the product gas is required at less than about 20,000 NCFH, because of the high cost of the adsorption equipment and the complex multiple valve and gas line systems required for efficient operation of such plants.
Single adsorption vessel adsorption systems can be operated in a manner such that the product gas is available on a continuous basis. This is accomplished by designing the system so that it produces sufficient gas product on an intermittent basis to meet the total product demand, and storing the product gas in a buffer tank to provide a continuous supply of the product gas at the desired pressure. U.S. Pat. Nos. 4,561,865 and 4,892,566 illustrate single vessel adsorption systems which have a surge tank to hold nonadsorbed product gas. The cycles described in these patents is relatively simple with a countercurrent depressurization step following the gas production step.
U.S. Pat. Nos. 5,370,728 and 5,658,371 disclose single vessel PSA systems which improve on the processes described in U.S. Pat. No. 4,561,865 and U.S. Pat. No. 4,892,566 in that they include a cocurrent depressurization step in which void space gas, i. e., nonadsorbed gas remaining in the adsorption vessel at the end of the adsorption step, is collected and stored in an equalization vessel for later use in purging and/or repressurizing the adsorption vessel in preparation for the next adsorption step. The processes of U.S. Pat. No. 5,370,728 and U.S. 5,658,371 are costly to operate because they require two gas storage vessels and associated piping and valves.
Adsorption processes and equipment configurations which are more efficient than those currently in use are continually sought. The present invention provides a single adsorption vessel-based process which increases the yield of nonadsorbed product gas and makes more efficient use of the partially fractionated void space gas, and provides a simpler single vessel adsorption system than currently available systems.